(1) Field of the Invention
The present invention relates to a structure of a motor which materializes an inexpensive permanent magnet motor adapted to be used in the field of industry, electronic appliances or automobiles, and being capable of outputting a high power coggingless toque.
(2) Description of Related Art
The so-called motor is used as a drive equipment which converts electric power into mechanical power in the field of industry, electric appliances and automobiles. World-wise demands for energy saving, the provision of motors having a high conversion efficiency are required. However, in addition of the enhancement of the efficiency, miniaturization of motors is also important, and accordingly, for example, magnetic motors utilize magnets each having a high energy product and containing a rare earth material, or a stator core having a higher coil density manufactured by a segmented core process.
However, coreless motors having no stator magnetic poles have to be used in a certain technical field since there would be caused a problem of pulsating torque, that is, the so-called cogging torque due to the relationship between the number of poles of magnets and the number of slots in a stator core in view of a high energy product of the magnets. In general, these motors have less cogging torque but has an energy conversion efficiency lower than that of permanent magnet motor each having a stator core.
The output torque of the coreless motor may be improved in such a way that the magnets are oriented in view of anisotropy thereof as disclosed in JP-A-2004-15906, JP-A-2004-56897 and JP-A-2005-20991. In this way, ring magnets which have been manufactured being magnetically isotropic are magnetized so as to change their magnetizing directions, but difficult problems have been encountered in the case of magnets having large thickness or in the case of magnetization in complicated magnetizing direction.
Further, in the case of magnets having a diameter exceeding 50 mm, the manufacture of ring magnets are difficult, and further, since a thermal expansion coefficient becomes negative in the case of ring magnets having a radial magnetizing direction, there would be caused such a problem that the magnets are broken or cracked due to temperature variation at a point of variation of the magnetizing direction, and so forth. Thus, usual ring magnets are protected at their outer surface with a binding tape containing glass fibers, carbon fibers or the like or with a thin metal wall cylinder made of stainless steel or the like, and so forth, after they are formed into a rotor. In the case of a magnet rotor formed of segment-like magnets other than ring magnets, that is, magnet segments bonded together for every pole, the outer peripheral surface of the rotor is in general protected with a binding tape or a metal cylinder as stated above.